The present invention relates to a synchronous position control apparatus and method and, more particularly, to a synchronous position control apparatus and method preferably applied to a scanning semiconductor exposure apparatus, machine tool and the like, that must maintain the synchronous relationship between the positions of two or more control targets at high precision.
In general, for example, a machine tool for mold processing must cut a free surface at high precision. This control is called contour control.
In contour control, the cutting performance of the machine tool is determined by the synchronous performance between positions along a plurality of axes serving as control targets attached to the machine tool.
In a normal position control system, a tracking error from a target position is processed as an important control index. In contour control, ensuring the synchronous characteristic between axes is more important than the tracking performance to a target position in realizing a high-performance apparatus. This will be exemplified.
FIGS. 12A and 12B show a conventional synchronous position control system for two orthogonal axes on the XY plane which assumes the XY table of a machine tool. FIG. 12A shows an X-axis position control system, and FIG. 12B shows a Y-axis position control system.
In FIGS. 12A and 12B, these position control systems have time-lags of first order with respect to target position command values 2a and 2b output from target position command value generation means (target position command value generators) 1a and 1b. Wox and Woy represent position loop gains 40a and 40b expressing the cutoff frequencies of these time-lags of first order.
FIG. 15 shows a locus on the plane when a straight path is drawn on the XY plane for Wox=Woy=5 Hz, i.e., the tracking locus of a low-gain synchronous position control system. FIGS. 16A and 16B show tracking errors along the X- and Y-axes at that time, i.e., the time response of the low-gain synchronous position control system.
FIG. 17 shows a locus on the plane when a straight line is drawn on the XY plane for high gains of Wox=10 Hz and Woy=12 Hz, i.e., the tracking locus of a high-gain synchronous position control system. FIGS. 18A and 18B show tracking errors along the X- and Y-axes at that time, i.e., the time response of the high-gain synchronous position control system.
For a low gain, as shown in FIGS. 15, 16A, and 16B, the tracking performance of the position control system is low, but its synchronous performance is high. For a high gain, as shown in FIGS. 17, 18A, and 18B, the tracking performance of the position control system is high, but its synchronous performance is low.
From the comparison between them, the system with a low response characteristic in FIGS. 15, 16A, and 16B exhibits a smaller path error from a target path regardless of a large tracking error.
As is apparent from this example, in a system in which synchronous performance for respective axes to be controlled is important, high control performance cannot be attained only by increasing the gain of the position control system and enhancing tracking performance. Synchronous control performance must be enhanced by making position control characteristics between control axes coincide with each other.
In position control of a machine tool which performs contour control, different servo characteristics along respective control axes impair the synchronous relationship between these axes. An error occurs between a target cutting path and an actual cutting path, resulting in low processing precision. To prevent this, servo characteristics along the control axes must coincide with each other as much as possible so as to maintain a synchronous relationship between these axes.
To realize a synchronous position control system capable of maintaining a synchronous relationship between respective axes, a position control system as shown in FIG. 13 is constituted in a conventional machine tool.
This servo system has a high-gain speed control loop 43, and a low-gain position control loop 44 outside the speed control loop 43. The servo system is built in this way in order to ensure system stability and suppress disturbance by the speed control loop, and ensure a response characteristic, and particularly, a synchronous characteristic, along each axis to a target value by the position control loop.
For example, when the control axes are three X-, Y-, and Z-axes in a mold processing unit, the gains of position control loops for the three axes must be the same, but the gains of speed control loops cannot always be the same. This is because mechanical structures for the respective axes are different owing to mechanical factors, and mechanical resonant points which determine the gains of the speed control loops are different.
To suppress disturbance, the gain of the speed control loop is desirably as high as possible. However, the gains of the speed control loops for control axes are generally different under limitations of mechanical resonance. Even if the gains of the position control loops are the same, the gains of the speed control loops for the respective axes are not always the same, and thus, the response characteristics along the respective axes to a target position value are different.
In the position control system of the machine tool, the gain of the position control loop is generally set as low as about {fraction (1/10)} the gain of the speed control loop. In this case, the characteristics of the speed control loop rarely appear on the response characteristic of the position control system to a target position value. For this reason, a control system having an arrangement as shown in FIG. 13 can build a synchronous position control system for each axis.
In the conventional synchronous position control system, however, the speed control loop serves as an internal loop of the position control loop, and the gains of these loops may interfere with each other.
That is, when the gain of the position control loop is set about {fraction (1/10)} or less the gain of the speed control loop according to the above method, the characteristics of the two control loops can be set substantially independently.
If the gain of the position control loop must be set high in terms of the response characteristic, the gains of the two control loops are difficult to maintain at greatly different values. In other words, if a high gain is set in the position control loop, the characteristics of the speed control loop appear as a response to a target position value. At this time, different characteristics of the speed control loops for respective axes deteriorate the synchronous position control characteristic.
In the conventional arrangement, the response characteristic to a target value is determined by the gain of the position control loop. In simple position control, which does not require synchronous position control, the settling time of the control system becomes long.
The response characteristic to a target value is approximated by a system of a time lag of a first order using the gain of the position control loop as a cutoff frequency, but is difficult to cope with a case in which response characteristics except for the first-order lag characteristic are desirable.
The position control system for determining synchronous performance forms a feedback system. This poses problems in system design such that a designer who does not know a control system is difficult to design a synchronous control system.
The present invention has been made to overcome the conventional drawbacks, and has as its first object to increase the degree of freedom for adjusting the response characteristic to a target value. It is the second object of the present invention to shorten the settling time in simple positioning control. It is the third object of the present invention to realize an arrangement which can facilitate the design.
To achieve the above objects, as the first synchronous position control apparatus of the present invention, a synchronous position control apparatus comprises position controllers which perform control based on position feedback control for a plurality of axes to be controlled, target position command value generators for respective axes which generate target position command values to be set in the respective position controllers, and converters for respective axes which convert, on the basis of predetermined transfer functions, the target position command values for the respective axes that are output from the respective target position command value generators, and setting the converted values in the respective position controllers, wherein characteristics of the transfer functions are set to substantially the same high-frequency cutoff characteristics for the respective axes, and the cutoff frequency of the transfer function is set to a smaller value than a servo band in any desired position controller.
As the first synchronous position control method of the present invention, a synchronous position control method of performing synchronous position control by setting target position command values for position control mechanisms for performing control based on position feedback control for a plurality of axes to be controlled comprises converting the respective target position command values for the respective axes on the basis of predetermined transfer functions, and setting the converted values in the respective position control mechanisms, and setting characteristics of the transfer functions to substantially the same high-frequency cutoff characteristics for the respective axes, and setting the cutoff frequency of the transfer function to a smaller value than a servo band in any desired position control mechanism.
In the prior art, a synchronous position control apparatus is constituted by position controllers having a relatively low response characteristic. In the present invention, a synchronous position control apparatus is constituted by position controllers having a high response characteristic. Further, filters (converters) having a high-frequency cutoff characteristic are inserted on the input stages of the respective position controllers. The characteristics of the filters are set to coincide with each other for all the control axes, and the cutoff frequencies of the filters are set lower than any position control loop gains for all the control axes.
When the filter is used on the input stage of the position control loop having a high response characteristic, the response characteristic to a position command value is substantially determined by the response characteristic of the filter. Therefore, by setting the characteristics of the filters to coincide with each other for all the control axes, response characteristics to position command values can substantially coincide with each other for all the control axes. A synchronous position control apparatus having high synchronous performance can be easily constituted.
Since the characteristics of the filter (converter) can be set independently of the stability of the control system, the degree of freedom for adjusting the response characteristic to a position command value can be increased. The filter which determines the response characteristic only converts a target position command value. In simple positioning control which does not require synchronous control, the target position command value can be supplied without the mediacy of this filter. Thus, the synchronous position control apparatus can be used as a system having high positioning performance.
As the second synchronous position control apparatus of the present invention, a synchronous position control apparatus comprises acceleration command value generators which generate target acceleration command values for a plurality of axes to be controlled, converters for the respective axes which convert the respective target acceleration command values in accordance with predetermined transfer functions, and outputting the converted values as converted acceleration command values, integrators for the respective axes which integrate the respective converted acceleration command values twice, and outputting the integrated values as position command values, compensators for the respective axes which perform predetermined compensation for the respective converted acceleration command values, and outputting the compensated values as feedforward signals, and position controllers which perform control based on position feedback control for the respective axes on the basis of the respective position command values and the feedforward signals, wherein characteristics of the transfer functions are set to substantially the same high-frequency cutoff characteristics for the respective axes, and the cutoff frequency of the transfer function is set to a smaller value than a servo band in any desired position controller.
As the second synchronous position control method of the present invention, a synchronous position control method comprises an acceleration command value generation step of generating target acceleration command values for a plurality of axes to be controlled, a conversion step of converting the respective target acceleration command in accordance with predetermined transfer functions for the respective axes, and outputting the converted values as converted acceleration command values, an integral step of integrating the respective converted acceleration command values twice, and outputting the integrated values as position command values for the respective axes, a compensation step of performing predetermined compensation for the respective converted acceleration command values, and outputting the compensated values as feedforward signals for the respective axes, and a position control step of performing control based on position feedback control for the respective axes on the basis of the position command values and the feed forward signals, wherein characteristics of the transfer functions are set to substantially the same high-frequency cutoff characteristics for the respective axes, and the cutoff frequency of the transfer function is set to a smaller value than a servo band in any desired position feedback control.
The second aspect adopts a feedforward loop for a target value to improve tracking performance to the target value. The structure of the compensator in the feedforward loop is simple and can be easily realized.